Generally the lifecycle of an apparatus utilizing power electronics, such as of a frequency converter, is determined by its capacitors, which are used in the DC-voltage intermediate circuit in particular for filtering noise voltages and also for storing electrical energy.
In frequency converters, large stresses are exerted on the capacitors of the DC-voltage intermediate circuit. Often unexpected destruction of a capacitor occurs, and it was not possible to anticipate its replacement. This further results in unwanted standstills in production and thus causes economic losses.
The ambient temperature, current ripple, high voltages and frequencies have a powerful impact on the lifetime of a capacitor. As a capacitor grows old its internal resistance increases, as also does the leakage current running through it, the capacitance decreases slowly over time. All these phenomena can be measured, but in practice they require opening of the appliance.
Manufacturers often define that a capacitor is at the end of its lifecycle when its capacitance has decreased by a certain relative amount.
The article E. C. Aeloiza; J-H Kim, P. Ruminot, P. N. Enjeti “A Real Time Method to Estimate Electrolytic Capacitor Condition in PWM Adjustable Speed Drives and Uninterruptible Power Supplies”, pp. 2867-2872, IEEE 2005, presents a method to estimate the state of an electrolytic capacitor in variable PWM alternating current drives and in UPS apparatuses, in which method the state can be examined at the place of usage. The method is based on the fact that the wear of a capacitor results in variations in its internal equivalent serial resistance (ESR). In the method in question, the average power of the capacitor unit, or the RMS value of the current over it, is measured. A drawback of the method is its complexity and the large requirement for the computational power of the control electronics used in the measurement.